Understanding the molecular mechanisms that contribute to the accelerated development of the diseases of aging is essential for healthy aging. Over the past 10 years, substantial evidence supports the notion that altered mitochondrial function and metabolic dysregulation play key roles. In this competitive renewal grant application of my first R01 as an independent PI, our overall goal is to identify how altered metabolism contributes to the diseases of aging and aging itself. We have made significant progress towards understanding how acyl-CoA species derived from metabolism induce protein modifications, and how mitochondrial sirtuin 5 removes them as a layer of metabolic control. Our body of work in the first 5-year funding period defines a new paradigm of protein acylation and deacylation, and identifies SIRT5 as a regulator of metabolism and nutrient homeostasis. In the course of these studies, we made the unexpected discovery that SIRT5 levels are physiologically regulated during normal cell cycle progression, and its absence leads to altered cell cycle control. This exciting finding identifies a long-sought-after condition under which sirtuin levels are controlled and is positioned to reveal the underlying biological role of this emerging regulator of aging. In this proposal, we will build upon these exciting preliminary data and focus on the following Specific Aims: Aim 1) interrogate the regulation of SIRT5 protein during cell cycle progression; Aim 2) determine how SIRT5 activity influences nutrient sensing; Aim 3) identify the physiological role of SIRT5 in skeletal muscle stem cells. Together, these studies combine an innovative conceptual framework and a comprehensive experimental design to determine the key biological role of SIRT5 in controlling specific nutrient-sensing responses. Furthermore, this study will build a foundation of knowledge to further understand how the metabolic state communicates with the cell cycle, and how loss of this communication contributes to the pathophysiology of aging. Ultimately, these studies will deepen our understanding of emergent, novel metabolic control mechanisms, and have the potential to inform the development of new therapies to maintain healthy aging.